A power amplifier is a key element in many electronic systems. For example, a power amplifier is one of the fundamental components of an electronic communication system, such as a wireless local area network (WLAN) or cellular phone system. The power amplifier and its operation can be a significant contribution to the cost and difficulty of design of the communication system. For example, excess use of power can impact the use of battery or other power source and contribute to difficulties in designing a communication system.
Problems in amplifier performance are gain compression and phase advance, due to the non-linearity of the amplifier. Designers have added biasing circuits to power amplifiers to improve performance over various conditions, yet these problems persist.
Many communication systems use two stage power amplifiers. FIG. 1 shows the typical fixed bias circuitry for a two-stage power amplifier using radio frequency (RF) chokes (inductors or resistors) and constant voltage supplies VB1, VB2. The RF impedances Z1, Z2 of these fixed bias circuits presented at the inputs of the first and second stage of the amplifier are constant when RF input power increases. The amplifier has gain compression and phase advance at high RF input power due to the non-linearity of the active devices under large RF drive. The gain compression is due to reduction of transistor transconductance (gm) at high RF input power. The phase advance is due to the base-collector capacitance variation at high RF input power. These gain compression and phase advance characteristics at high RF input power result in poor linearity performance of a power amplifier.
Because of the problems experienced with power amplifiers and their use, improved circuits and systems are needed.